It has become increasingly more clear that Bacteroides melaninogenicus is an essential component in some mixed anaerobic infections, particularly some types of periodontal disease. Our studies are directed toward elucidation of the role of B. melaninogenicus in the pathogenesis of periodontal disease. Inasmuch as the bacterial cell surface is exposed to the host, we have chosen to study the immune response to outer cell membrane antigens. After purification of the antigens we have characterized them chemically and immunologically. We are using these antigens to measure the local and systemic humoral immune response to infections caused by B. melaninogenicus by means of sensitive serologic assays. After clarifying the immune response to these bacterial immunogens, we propose to investigate the potential for vaccination with the antigens in an effort to protect against infection by B. melaninogenicus. The purified antigens will be used also as a means of serogrouping this relatively complex bacterial species, and to develop diagnostic serologic tests.